This invention relates to an improvement in a process for the synthesis of cryolite by reaction between sodium aluminate and a fluoride solution which contains silica as an impurity dissolved therein.
Various processes are known for the production of synthetic cryolite which is largely used in the extraction of aluminum metal from alumina by electrolysis. An old practice was the use of high purity fluorite as starting material, but most of the recent processes utilize certain fluorides obtained as by-product in the production of phosphatic materials.
At present, the most prevailing method for industrial synthesis of cryolite is to add sodium aluminate to an aqueous solution of ammonium fluoride and/or sodium fluoride prepared by alkali decomposition of a silicofluoride obtained as by-product of wet process phosphoric acid. The following equations are representative of reactions according to this method. Among these, primary importance and preference have been given to the process represented by Equations (A-1) and (A-2), i.e. the use of a solution of ammonium fluoride and sodium fluoride prepared by ammonia decomposition of sodium hexafluorosilicate (simply called sodium silicofluoride), though the processes represented by Equations (B-1), (B-2); and (C-1), (C-2) are also practicable. EQU Na.sub.2 SiF.sub.6 +4NH.sub.3 +2H.sub.2 O.fwdarw.4NH.sub.4 F+2NaF+SiO.sub.2 .dwnarw. (A-1) EQU 4NH.sub.4 +2NaF+NaAlO.sub.2 .fwdarw.Na.sub.3 AlF.sub.6 +4NH.sub.3 +2H.sub.2 O (A-2) EQU H.sub.2 SiF.sub.6 +6NH.sub.3 +2H.sub.2 O.fwdarw.6NH.sub.4 F+SiO.sub.2 .dwnarw. (B-1) EQU 6NH.sub.4 F+NaAlO.sub.2 +2NaOH.fwdarw.Na.sub.3 AlF.sub.6 +6NH.sub.3 +4H.sub.2 O (B-2) EQU Na.sub.2 SiF.sub.6 +6NaOH.fwdarw.6NaF+Na.sub.2 O.multidot.SiO.sub.2 +3H.sub.2 O (C-1) EQU 6NaF+NaAlO.sub.2 +2H.sub.2 O.fwdarw.Na.sub.3 AlF.sub.6 +4NaOH (C-2)
As represented by Equation (A-1), silica is formed during ammonia decomposition of sodium silicofluoride. Although the precipitated silica can be removed from the fluoride solution, it is inevitable that a certain amount of silica remains dissolved in the fluoride solution. As a matter of inconvenience, most of the silica dissolved in the fluoride solution is liable to shift into the cryolite formed by the reaction of, for example, Equation (A-2). This raises a serious problem since the presence of silica in cryolite used as the bath material in electrolytic extraction of aluminum causes significant lowering of the current efficiency.
Therefore, various methods have been proposed to reduce silica content in synthetic cryolite. For example, Japanese Patent Specification Publication No. 47(1972)-3445 proposes to remove not only phosphorus matter but also silica from an industrially obtained fluoride solution by treatment with a ferric salt, and U.S. Pat. No. 2,916,352 proposes to accomplish a reaction to form cryolite from a fluoride containing phosphoric acid under a strongly acidic condition expressed by pH values below 2.0. However, none of the hitherto proposed methods is fully satisfactory when consideration is given both to the efficiency of reducing silica content in the synthesized cryolite and to the costs of material and operation of the process.
As another problem about the above described process, the synthesized cryolite exhibits an unsatisfactorily large ignition loss. When such a synthetic cryolite is used in an electrolytic bath for the extraction of aluminum, a considerable amount of fluorine is dissipated from the heated cryolite and offers an air pollution problem. Therefore, it is a usual practice to fire the synthesized and dried cryolite at 350.degree.-700.degree. C. before marketing the synthetic cryolite. However, this means the addition of an extra and theoretically unnecessary procedure to the synthesis process and offers a disadvantage that a considerable amount of fluorine is dissipated at this stage. Concerning the reactions represented by the above Equations, Japanese Patent Specification Publication No. 53(1978)-7158 shows reaction conditions favorable to the synthesis of cryolite of small ignition loss. However, it is considered that still there is a much room for improvement on this point.